The present invention relates to an electrodeionization apparatus for electrically separating ions from water and a method of operating the same.
Electrodeionization apparatuses have been used in the field of pure water and ultra pure water production. A plate and frame type electrodeionization apparatus includes an anode, a cathode, anion-exchange membranes and cation-exchange membranes. The membranes are arranged alternately in such a manner as to alternately form concentrating compartments and desalting compartments (dilution compartments) between the anode and the cathode. The desalting compartments are filled with an ion exchanger such as an ion exchange resin. Raw water, which is to be treated, is introduced into the desalting compartments of the electrodeionization apparatus. Ions contained in the water permeate through the ion exchange membranes into the concentrating compartments.
Japanese patent publication S56-16688B discloses an electrodeionization apparatus comprising a cylindrical casing, electrodes (anodes and cathodes) having a shape of a rod and being arranged parallel with each other, and cylindrical ion-exchange membranes surrounding the respective electrodes. Each ion-exchange membrane defines a concentrating compartment surrounding the electrode so that deionized water flows outside of the respective ion-exchange membranes.
In this conventional electrodeionization apparatus, concentrated water from the concentrating compartments surrounding the anodes and concentrated water from the concentrating compartments surrounding the cathodes are mixed and circulated to the respective concentrating compartments.
According to such a conventional electrodeionization apparatus, it is difficult to remove silica at a high removal rate due to diffusion of silica from the concentrating compartments. For instance, when the electrodeionization apparatus is operated under conditions in which the feed water contains a silica concentration of 200 parts per billion (ppb) and the overall product water recovery from the apparatus is 90%, water flowing the concentrating compartments contains a high silica concentration about 2000 ppb. As a result, the silica diffuses from the concentrating compartments to the desalting compartments at a high rate, thus, the silica concentration in the product water flowing out of the apparatus fails to become sufficiently low.
It is an object of the present invention to provide an electrodeionization apparatus and method which overcomes the foregoing problems.
It is a further object of the present invention to provide an electrodeionization apparatus which restrains the diffusion of silica from the concentrating compartments, producing product water with an extremely low silica concentration.
The method of the present invention employs an electrodeionization apparatus which has at least one anode, at least one cathode, and ion-exchange membranes which are arranged to form at least one concentrating compartment and at least one desalting compartment between the anode and the cathode. Concentrated water flows into the concentrating compartment, feed water flows into the desalting compartment, and the product water flows out of the desalting compartment. The silica concentration of the concentrated water flowing out of the concentrating compartment is less than 1000 times as great as that of the product water.
In order to produce the product water containing silica at a concentration of 0.1 ppb or less, the concentrated water flowing out of the concentrating compartment contains the silica preferably at a concentration of less than 100 ppb, more preferably less than 70 ppb.
In an aspect of the invention, the electrodeionization apparatus has a plurality of concentrating compartments along the flow direction of water in the desalting compartment, and the concentrated water flowing out of the concentrating compartment which is located at the most downstream side in regard to the desalting compartment has the silica concentration less than 1000 times as great as that of the product water. According to this aspect of the present invention, the gradient of silica concentration from the concentrating compartment to the desalting compartment is small even near the outlet of the concentrating compartment, so that silica is restricted to diffuse from the concentrating compartment to the desalting compartment, and the product water contains silica at an extremely low concentration.
In another aspect of the invention, the number of the concentrating compartments is n (n is an integer of 2 or more), i.e. first through n-th concentrating compartments are provided along the flow direction of water in the desalting compartment, a part of concentrated water flowing out of the first concentrating compartment is discharged and the remainder of the concentrated water is circulated into the first concentrating compartment, a part of concentrated water flowing out the k-th (2xe2x89xa6kxe2x89xa6n) concentrating compartment is supplied to the (kxe2x88x921)-th compartment just ahead of the k-th compartment and the remainder of the concentrated water is circulated into the k-th concentrating compartment, and a part of the product water is supplied to the n-th concentrating compartment which is located at the most downstream side in regard to the desalting compartment. According to this aspect of the present invention, the silica concentration is reduced sufficiently in the concentrated water in the concentrating compartment nearest the outlet and in the product water.
In the present invention, water to be added to the concentrating compartment can be supplied to a feed line connected to the concentrating compartment. Instead thereof, the water can be supplied directly to the concentrating compartment.
The electrodeionization apparatus according to an aspect of the present invention has an anode, a cathode, and ion-exchange membranes so that a concentrating compartment and a desalting compartment are defined by the ion-exchange membranes relative to the anode and the cathode. The concentrated water flows through the concentrating compartments, the feed water flows into the desalting compartment and the product water is taken out of the desalting compartment. The electrodeionization apparatus has first through n-th concentrating compartments from the most upstream side to the most downstream side in regard to the desalting compartment.
According to an aspect of the invention, the electrodeionization apparatus has a supplier which supplies a part of the product water to the n-th concentrating compartment. The product water may be fed to the concentrating compartment directly or to a line for feeding the concentrated water to the n-th concentrating compartment.
The electrodeionization apparatus may have a flow line or lines which discharges a part of concentrated water flowing out of the first concentrating compartment, circulates the remainder thereof into the first concentrating compartment, supplies a part of the concentrated water flowing out of the k-th (2xe2x89xa6kxe2x89xa6n) concentrating compartment to the (kxe2x88x921)-th concentrating compartment, and circulates the remainder of thereof into the k-th concentrating compartment.
Each desalting compartment may be packed with an ion exchanger such as ion exchange resin or ion exchange fiber. The concentrating compartment may be packed with an ion exchanger or activated carbon. The number xe2x80x9cnxe2x80x9d is preferably 2 to 10, more preferably 2 to 5 most preferably 2 to 4. Other aspects of the invention will be described later with the preferred embodiments.
An electrodeionization apparatus according to another aspect of the invention has at least one anode, at least one cathode, ion-exchange membranes arranged between the anode and the cathode; concentrating compartments and at least one desalting compartment being defined by the ion-exchange membranes. The concentrated water flows through the concentrating compartments, feed water is fed into the desalting compartment, and product water is taken out from the desalting compartment. The concentrating compartment is divided into a plurality of concentrated water flowing sections by a partition extending along a direction crossing to the flowing direction in the desalting compartment. The concentrated water flows in the concentrated water flowing sections along a direction crossing the flow direction in the desalting compartment.
In the electrodeionization apparatus according to the above described aspect, the number of the concentrated water flowing sections is n which is an integer of at least 2, so that first through n-th concentrated water flowing sections are provided along the flow direction of water in the desalting compartment. A part of the concentrated water flowing out said first concentrated water flowing section is discharged and the remainder of the concentrated water is circulated to the first concentrated water flowing section. A part of concentrated water flowing out the k-th concentrated water flowing section, wherein 2xe2x89xa6kxe2x89xa6n, is supplied to the (kxe2x88x921)-th flowing section just ahead of the k-th flowing section and the remainder of said concentrated water is circulated to the k-th concentrated water flowing section. A part of the product water is supplied to the concentrated water flowing section which is located at the most downstream side.
An electrodeionization system according to still another aspect of the invention has a plurality of electrodeionization apparatuses, and the number of the plurality of electrodeionization apparatuses is n which is an integer of at least 2. Water to be deionized flows from the first electrodeionization apparatus toward the n-th electrodeionization apparatus in series, and product water is taken out of the n-th electrodeionization apparatus. Each of the electrodeionization apparatus has at least one anode, at least one cathode, ion-exchange membranes arranged between the anode and the cathode; concentrating compartments and at least one desalting compartment being defined by said ion-exchange membranes. The concentrated water flows through the concentrating compartments, feed water is fed into the desalting compartment, and product water is taken out from the desalting compartment. The system has a flow line for supplying a part of said product water to the concentrating compartment of the n-th electrodeionization apparatus. The system has a flow line for discharging a part of concentrated water flowing out of the concentrating compartment of the first electrodeionization apparatus and circulating the reminder of the concentrated water into the concentrating compartment of the first electrodeionization apparatus. The system further has a flow line for supplying a part of concentrated water flowing out of the concentrating compartment of the k-th electrodeionization apparatus, wherein 2xe2x89xa6kxe2x89xa6n, to the concentrating compartment of the (kxe2x88x921)-th electrodeionization apparatus and circulating the remainder of the concentrated water into the concentrating compartment of the k-th electrodeionization apparatus.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.